1. Field of the Invention
The present invention relates to an improved process for preparing strongly water-absorbing and water-swellable polysaccharide-(meth)acrylic acid graft polymers. The present invention also relates to the polysaccharide-(meth)acrylic acid graft polymers produced by this process.
2. Discussion of the Background
Water-absorbing polymers have a number of uses in the sanitation and health fields as water absorbents in paper diapers and towels, tampons, hospital bedding, electrolyte thickeners in dry cells, moisture-holding media or water reservoirs in agriculture, and as drying agents. In these applications, both the absorption capacity and the rate of absorption are important.
Suitable polymers are derivatized polysaccharides, generally grafted with water-soluble vinyl monomers, such as carboxymethylcellulose, hydrolyzed starch-acrylonitrile graft polymers, acrylic acid-starch graft polymers, or fully synthetic, slightly crosslinked polymers such as partially crosslinked polyacrylic acid salts or partially crosslinked polymaleic acid derivatives.
Processes for preparing polysaccharide graft polymers by grafting (meth)acrylic acid on starch in an aqueous solution are disclosed by Japanese Patents 40-43,408 and 80-139,408. However, the following difficulties arise in the crosslinking polymerization of water-soluble monomers in aqueous solution to prepare particulate gels:
(1) Water-swollen gels that are difficult to handle are obtained as reaction products. The solid has to be separated from the aqueous phase by precipitation, so that it occurs in a lumpy form. It then must be dried and ground.
(2) Mixing and stirring of aqueous solutions are not possible even with low solids contents of 10 to 20% because of the swelling of the graft polymers and high viscosity. For this reason, a controlled reaction cannot be carried out in conventional stirring equipment.
(3) Effective grafting of water-soluble monomers on starch to form gels containing graft polymers with a branched-chain structure necessary for good water absorption capability is not possible in purely aqueous reaction media, since water-soluble monomers like acrylic acid or acrylamide polymerize here in an unavoidable side reaction to form water-soluble ungrafted homopolymers, which have only a thickening action but are not water-swellable, and therefore impede the water-absorption capability.
It has therefore been attempted to prepare polysaccharide graft polymers by suspension polymerization.
An inverse suspension polymerization (water-in-oil suspension polymerization) for grafting polysaccharides with water-soluble monomers is described in DE-OS 28 40 010. The process has the following features:
(1) The monomers are added to the organic phase in the form of an aqueous solution that is prepared in a separate reaction vessel, together with a relatively high additional quantity of a preferably ionic water-soluble surfactant. The surfactant here is considered to be necessary for homogeneous distribution of the aqueous phase in the organic phase. This so-called semicontinuous procedure thus requires an additional reaction vessel which has to be equipped with a cooler to carry off the heat of neutralization when using acrylic acid as the monomer.
(2) Mixtures of ionic and nonionic surfactants are preferably used in large additional amounts of from 0.5 to 12% based on the solvent.
(3) As shown in the examples, the water-soluble monomers, acrylic acid and acrylamide, are preferably used in such quantities that water-soluble polymers are obtained, and not water-swellable, gel-like graft polymers.
(4) No crosslinking agents are used.
Japanese Patent 80-161,813 describes the preparation of a water-absorbing graft polymer by grafting a polysaccharide with acrylic acid in an aliphatic hydrocarbon as solvent in the presence of an organophilic nonionic surfactant and a water-soluble initiator. However, the product formed in the polymerization has a tendency to clump.
In European Patent 36,463 A2, an oil-soluble surface-active substance with a relatively high hydrophil-lipophil balance (HLB value) of 8 to 12, for which sorbitan monolaurate is preferred, is used in the inverse suspension polymerization of acrylates without polysaccharide. Severe gel formation and clumping occur during the polymerization. The products are distinguished by high water absorption capacity. The cost of equipment is relatively high.
DE-OSS 33 31 644 and 35 07 775 describe water-in-oil suspension polymerizations for the preparation of polyacrylate powders, for which oil-soluble protective colloids or surfactants with low HLB values must be used. After partial dehydration and crosslinking, polymers are obtained that contain no polysaccharide. The water absorption capacity is satisfactory. The polymerization is carried out by a continuous process with relatively high costs for equipment, with the aqueous monomer solution being prepared by neutralization of acrylic acid with an aqueous solution of alkali metal hydroxide or ammonium hydroxide with cooling in a separate reaction vessel, and being fed at a controlled rate to the organic solution.
Uncrosslinked polysaccharide graft polymers are prepared by inverse suspension polymerization in DE-OS 36 13 309. Polar and nonpolar nonionic surfactants are added in this case. Powdered graft polymers with high water absorption capacity are obtained. However, not all of the needs are met for products requiring high suction power, such as diapers and towels.